Work is in progress on various aspects of the metabolism of the red blood cells of man and different animals. Studies are being carried out on the intermediate metabolites of red cells of alpacas living at high altitude and at sea level in order to find out more about the adaptations of these animals to their low oxygen native habitat. Detailed analyses are continuing of the energy metabolism of reticulocytes which are obtained from the circulating blood of rats and rabbits after bleeding or treatment with phenylhydrazine. Research is also in progress on the changes which take place in the metabolism of soluble nucleotides and related purine metabolites in response to acute hemorrhagic hypoxia, in rabbits, and on the effects produced by transfusion of the hypoxic animals with fresh and stored red cells.